In a conventional floating roof tank, there is provided a clearance space or rim space between the tank side wall and the vertical rim or edge of the roof. It is necessary to provide a clearance space to permit unrestrained vertical travel of the roof within the tank. The clearance space is of sufficient size so that local dimensional variations in the circularity of the tank sidewall or shell, commonly called out-of-roundness, which can result from uneven foundation settlement, imprecise fabrication or erection, or unusual live loads such as high winds and the like, do not hamper vertical travel of the roof.
One apparatus used to maintain the roof centered in the tank and to effect a seal against evaporation loss uses a vertical, metallic shoe adapted to slidably contact the entire circular vertical inner side wall of the tank and means supported by the roof, such as pantagraph hangers, for pressing the shoe against the inner side wall, as well as to support the shoe. Vapor loss between the roof and shoes is prevented by a flexible nonpermeable fabric barrier which extends from the upper part of the shoe to the floating roof top edge. Such apparatus is disclosed in many U.S. patents including U.S. Pat. Nos. 2,587,508; 2,630,937; 2,649,985 and 2,696,930.
Although such types of seals function quite well in commercial installations, some product vapor may still escape. One way in which vapor may escape is through the top of the flexure joints in the vertical shoe.
One type of vertical metal shoe which is widely employed has a plurality of spaced apart vertical flexures which permit the shoe circumference to increase or decrease within predetermined limits. The flexures are in the form of an integral open ended trough which terminates below the shoe top edge. The trough longitudinal opening will generally be substantially flat against the tank wall. Goldsby et al U.S. Pat. No. 2,586,813 discloses a floating roof tank with a rim space seal using a shoe having such flexures. When product vapor forms in the annular space between the shoe and floating roof, and conditions are right, some emissions may flow out through the small opening in the shoe above the upper ends of the flexure troughs, it was proposed as early as the Goldsby et al patent, supra, to plug the top of the flexures with a neoprene sponge and to put a flap over it. In addition, a sheet of neoprene was to be placed over any adjacent joints or gaps. Later, a one piece cover cap was put in use for the described purpose. Although these systems for sealing the flexure tops have been helpful in suppressing vapor emissions, the desire to lower even further vapor emissions into the atmosphere has resulted in a need to have available alternative flexure closures which are suitable for preventing vapor emissions.